Which indefinite article would work best in the following sentence?

on the beach, he built huge sandcastle.

a.
a

b.
an

c.
the

How did magellan's discovery of what is now considered the philippines directly affect the native populations that were living there? a. they disappeared deep into the new land and started another settlement b. they became aggressive after having a new religious view upon them c. they became restless to travel the world on the ships they encountered d. they became angered over losing their lands' raw goods like fruits and spices